Comparison of recirculation and drain-to-waste hydroponic systems in relation to medical cannabis (Cannabis sativa L.) plants

Abstract

Rising temperatures, higher water consumption for irrigation, and the discharge of agricultural wastewater may soon adversely affect water resources across the entire planet and its people. Agriculture consumes large amounts of water, and there is a pressing need to explore possibilities for recirculation and reuse of nutrient solutions to reduce the associated environmental and economic costs of hydroponics. This study compared two different nutrient systems for hydroponic cultivation (recirculation, RS, and drain-to-waste system, DS), and assessed their effect on the growth and concentration of elements and secondary metabolites in medical cannabis (Cannabis sativa L.) plants. The dry decomposition method with flame atomic absorption spectroscopy (FAAS) and inductively coupled plasma optical emission spectroscopy (ICP-OES) were used for the elemental analysis of plant organs. Nitrogen content was measured using the Kjeldahl method. A high-performance liquid chromatograph equipped with a diode array detector (HPLC-DAD) was used to analyze the phytocannabinoid profile in ethanolic flower extracts. Gas chromatography with flame ionization detection (GC-FID) and gas chromatography with mass spectrometry (GC-MS) were employed to determine the content of terpenic compounds in hexane extracts. Cultivation of plants with the RS system, achieved an 87% higher maximum yield of the main cannabinoid of medical cannabis chemotype I, tetrahydrocannabinolic acid (THCA), with lower total water and nutrient consumption, but with a longer cultivation time and a lower concentration of the sesquiterpene, β-caryophyllene. In contrast, the DS system enabled better control over the nutrient solution with stable delivery of the exact concentrations of nutrients. DS accelerated plant maturation, but at the cost of higher water and fertilizer consumption and a significantly lower total yield of THCA and monoterpenes.